It is shown experimentally that the main processes first causing breakdown of the sliding solid contact (SSC) carrying large currents are not two-dimensional processes of the velocity skin-effect type, but pinch instabilities developing in the contact interface. Electromagnetic and electrothermal explosive ejection of ionized material of low-mass pinch waists ignite parasite-shunting arcs behind and ahead of the armature under acceleration. Thus one has to speak of transition into arcing mode not of the SSC alone, but of the launch process as a whole.It has been noted that during the transition into the arcing mode the solid armature begins to behave as a hybrid armature of pulling type. At this operation mode, two stable arcs appear, due to pinch cords moving forward along the contact surfaces, which are fixed to both contact surfaces of the armature near their leading edges. Consequently, the armature main body is accelerated under the action of internal tensile forces originating in its leading part where the I × B forces are applied.

Our objective was to elucidate the physical reasons of the velocity limitation in the plasma armature (PA) railgun at the 6 k d s level. To do this, we have designeda simple system having no vacuum and no preaccelerator. The 1-2 g mass projectile is launched at the maximum possible constant acceleration4he limit of the projectile strength or the electrothermal explosion of rail surfaces, while making use of the effects of the material strengthening under the great confining pressure and rail current redistribution due to the presence of conducting shields. Such an approach provides the maximum possible energy efficiency of the launcher at its minimum conceivable length (as short as 50-80 cm). This greatly simplifies the design and fabrication but leads inevitably to the idea of the "fixed round"-"one assemblyone shot". Our experiments have confirmed the known hypothesis that the cause of the velocity saturation at the 6 k d s level is drag and decay of PA o'Wing to the increase of the inductive counter-emf with velocity and overload of PA with the ablated matter. The use of a compacted PA allowed us to overcome the "6 km/s limit" and to launch a 1 g projectile to 7.1 k d s in a bore length of 56 cm at a total efficiency of 10%. We believe that the simplest way to furtber velocity increase, while taking advantage of the compacted PA, is to increase the barrel length along with a corresponding increase of the energy stored. discussed below in a comp'uison with our experimental results which shed some light on the reasons for the "6-km/s limit". This will be followed by experiments carried out with acomp'xt PA which demonstrate the possibility of overcoming this limit.Our dztzz suggest that the velocity saturation is un'mbiguously due to the PA sprmding along the b'mel, so that to force the plasma to function properly, one should maintain the PA in the compact sme.In the ideal case of neglecting the losses for heating, wall ablation, friction etc. EM speed-up of a body in a railgun at a constant current I ('and, accordingly, constant acceleration a) is described by simple relations [4] Here V is the final projectile velocity, L' = const is the electrode unit length inductance, At is the acceleration time, 'and Q = IAt is the total electric charge consumed in At. This velocity is rtxzched at the lengthI . THE "G-KM/S LIMIT" IN PLASMA ARMATURE S = V A t / 2 = L'Q2/4m (2) RAILCUN LAUNCHING which paradoxically does not depend on acceleration time 'and ( h l y limited progress in EM 'acceleration of solid bodies has been reported in the fifteen years after the pioneering work of Rashleigh and Marshall [l] who accelerated a lexan projectile of 3 g in mass to 5.9 km/s in a 4 m-long railgun by means of a plasma armature (PA). Despite a comprehensive thwretical development of the EM launch problems 'and somewhat contradictory 'announcements of a velocity 8.5 km/s rexhed with m = 2 g [2], the published results obtained by other researchers remain at the 1978 level. An increase of the energy input and variation of the launc...

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